RP* 04 Determination of Young's Modulus Flashcards
Define Young’s Modulus.
The Young’s Modulus of a material its ratio of tensile stress to tensile strain. It is a measure of a material’s stiffness.
How is stress calculated?
Stress = Force / Cross-Sectional Area
How is strain calculated?
Strain = Change in Length / Original Length
What is the unit of stress?
Pascals (Pa) or Nm⁻²
What is the unit of strain?
Strain is unitless since it is a ratio of two lengths.
What is the unit of Young’s Modulus?
Pascals (Pa) or Nm⁻²
How can the cross-sectional area of a thin wire be measured?
The wire’s diameter can be measured. It should be measured in several places along the wire, using a micrometer. The average diameter can then be used to calculate a mean cross-sectional area.
What safety precaution should be taken when stretching thin wires?
Safety goggles should be worn since the wire may snap when under a tensile load and could cause an eye injury.
Why should the temperature of the surroundings be kept constant when carrying out this experiment?
Metals undergo thermal expansion when there is a temperature increase, and this would slightly change the dimensions of the wire.
Why should a pre-stress be applied to the wire when setting up this experiment?
A pre-stress should be applied so that all kinks in the wire are removed and the wire is taught, before measurements are taken.
How can the Young’s Modulus be determined from a graph of extension against load?
The gradient of the graph is e/F
E= L/(A x Gradient)
(Note that the order of the axes of the graph is important here.)
Suggest what has happened if the length of the wire doesn’t return to its original length when unloaded.
If the wire doesn’t return to its original length when unloaded, the load may have exceeded the wire’s elastic limit and consequently the wire has undergone plastic deformation.
How can the load applied on a wire be calculated from the mass added to the end of the wire?
Load = Mass x Gravitational Field Strength F = mg
What safety precaution should be taken when using hanging masses?
Never stand with your feet below the hanging masses in case the wire snaps and the masses fall. It is good practice to place a padded bucket below them.
Suggest how the extension of the wire may be measured.
A marker, such as a small piece of tape, could be added to the wire. A ruler could then be placed underneath the wire, allowing the movement of the marker to be measured.
